Sorry, but I agree with wild spot, doesn't sound like he was all that rock solid. Its not your fault, I don't expect a horse to be beginner safe within one summer of riding. I've had horses that I would put a beginner on after that period, but not completely turn them over to.
Remember, anytime that anyone is with a horse, they are training it, either well, or poorly. I always have to sigh and shake my head when I pop on my schoolies that were at one point extremely fine tuned. However, I can't blame the students, they don't have near the years or experience under their belts like I do, that's why they are taking lessons, they will all make mistakes and it does take a while to develop the feel and the confidence. Sadly, the horses, and the people who trained them, are the ones that pay for it. Just remember, at one point, you were that beginner child. Years of training can be undone in no time at all. Poor holding patterns and postural issues can develop in a horse that previously seemed unshakeable in a short matter of time. I'm sorry that you had this happen, but you shouldn't be surprised.